Lab raised sea urchins to pioneer efforts to reverse coral reef decline

Scientists from the University of Miami, the University of North Carolina at Wilmington, and the National Oceanic and Atmospheric Administration (NOAA) Southeast Fisheries Science Center have coordinated the release of Diadema antillarum sea urchins on an experimental site at Little Grecian Reef in a Florida Keys National Marine Sanctuary. The creatures are critical to coral reef renewal because they eat coral-smothering algae.

Coral reefs throughout the Caribbean and Florida have undergone major degradation over the past two decades. Among the causes of decline are the loss of the previously abundant, grazing Diadema, in 1983 by an epidemic which killed 95% of the population throughout the entire region. This was followed a few years later by increasing losses of live coral cover due to elevated seawater temperatures that cause corals to ‘bleach’. This phenomenon is thought by some to be one of the first early-warning signs of global warming.

Without the large numbers of sea urchins to graze the algae down, the reef substrate has become progressively overgrown by fleshy seaweeds instead of the short turfs and crustose coralline algae that characterize healthy, heavily grazed coral reefs. The thick cover of seaweeds has prevented tiny coral larvae from recolonising the reef substrate. Diadema has begun to recover in some locations in the Caribbean and these locations are beginning to show signs of reduced algal cover, but as yet the sea urchin shows little evidence of returning to the coral reefs of the Florida Keys, which have lost 50% or more of their coral cover over the last 10 years.

After the release of the juvenile Diadema researchers will closely monitor their behaviour and health as there is a major concern that fish and invertebrate predators will try to eat the sea urchins. The second step in the restoration effort will be to raise millions of coral larvae from field-collected spawn, and then entice the larvae to settle onto the reef areas where the Diadema were re-introduced. The reef-building corals targeted for restoration mass spawn during a few brief days each summer dictated by the lunar cycle and new research has allowed scientists to predict the spawning dates, permitting teams of divers to collect the spawn as they are released by the corals.

The team will be using large floating nursery chambers developed by colleagues working on the Great Barrier Reef to raise the coral larvae until they are mature enough to put down on the reef. However, even if all goes according to plan, it will take years before the scientists see the fruit of their labours as reef-building corals grow only a centimetre or less per year and full recovery will take decades to centuries.